1. Field of the Invention
The present invention relates to a nonaqueous electrolyte secondary battery.
2. Description of the Related Art
In recent years, a nonaqueous electrolyte battery using lithium as the negative electrode active material attracts attention as a high energy density battery, and a primary battery using, for example, manganese dioxide (MnO2), carbon fluoride [(CF2)n], or thionyl chloride (SOCl2) as the positive electrode active material are already in wide use as a power source for desk-top computers, watches, and as memory back-up batteries. Further, with progress achieved in recent years in miniaturization and weight reduction in various electronic appliances such as VTRs and communication appliances, the demands have been increased for a secondary battery having a high energy density for use as the power source for such appliances. Much research is thus being conducted on a lithium secondary battery using lithium as the negative electrode active material.
Specifically, research is being conducted on a lithium secondary battery comprising a negative electrode containing lithium, an electrolyte selected from the group consisting of a nonaqueous electrolysis solution and a lithium conductive solid electrolyte, and a positive electrode containing as a positive electrode active material a compound capable of carrying out a topochemical reaction with lithium. Incidentally, the nonaqueous electrolysis solution used is prepared by dissolving a lithium salt such as LiClO4, LiBF4 or LiAsF6 in a nonaqueous solvent such as propylene carbonate (PC), 1,2-dimethoxy ethane (DME), γ-butyrolactone (γ-BL) or tetrahydrofuran (THF). Compounds capable of carrying out a topochemical reaction with lithium include TiS2, MOS2, V2O5, V6O13, and MnO2.
However, the lithium secondary battery outlined above has not yet been put in to practical use. It should be noted in this connection that lithium used in the negative electrode is pulverized after the secondary battery is repeatedly subjected to the charge-discharge operation. As a result, the lithium is converted into a highly-reactive lithium dendrite, which impairs the safety of the secondary battery. Also, related damage, short-circuiting and thermal runaway of the battery tend to be brought about. In addition, the charge-discharge efficiency is lowered, which shortens the cycle life. Such being the situation, the lithium secondary battery outlined above has not yet been put into practical use.
Under the circumstances, it is proposed to use a carbonaceous material capable of absorbing-desorbing lithium, such as coke, a baked resin, a carbon fiber or a vapor-grown carbon in place of the metal lithium. The lithium ion secondary battery that has been commercialized in recent years comprises a negative electrode containing a carbonaceous material, a positive electrode containing LiCoO2, and a nonaqueous electrolyte. In this lithium ion secondary battery, a further improvement in the charge-discharge capacity per unit volume is required in accordance with the demands for the further miniaturization of electronic appliances and for the continuous use of the secondary battery over a longer period of time. Such being the situation, vigorous research is being conducted in an attempt to develop a lithium ion secondary battery meeting these requirements. However, a sufficiently satisfactory result has not yet been obtained. Therefore, it is necessary to develop a new negative electrode material for commercializing a secondary battery having a larger capacity.
It is proposed to use an elemental metal such as aluminum (Al), silicon (Si), germanium (Ge), tin (Sn), or antimony (Sb) as a negative electrode material that permits obtaining a capacity larger than that obtained by a carbonaceous material. Particularly, in the case of using Si as a negative electrode material, it is possible to obtain a large capacity, i.e., a capacity of 4,200 mAh per unit weight (1 g). However, in the case of using a negative electrode formed of the elemental metal exemplified above, the bond between the adjacent metal atoms is broken due to the repetition of the absorption-desorption of Li, which leads to fine pulverization of the negative electrode, resulting in failure to obtain high charge-discharge cycle characteristics.
Under the circumstances, it is attempted to improve the charge-discharge cycle life of the secondary battery by using as the negative electrode material an alloy containing element T1 that does not form an alloy with lithium, such as Ni, V, Ti or Cr and element T2 forming an alloy with lithium. Also, in order to suppress the pulverization of the negative electrode material, which causes the deterioration of the cycle characteristics of the secondary battery, it is attempted to suppress the volume expansion by dispersing, for example, a phase reactive with lithium such as an element T2 phase, and a phase that is inactive with lithium, such as an element T1 phase in a nano scale, or by making the entire alloy phase amorphous.
In any of the negative electrode materials described above, an alloying reaction is carried out between the negative electrode material and lithium so as to permit lithium to be absorbed by the negative electrode material. The initial charging reaction is as exemplified by reaction formula (A) given below:T1xT2y+Li→xT1+LiT2y  (A)
The second charge-discharge reaction et seq. after the initial charge-discharge reaction proceeds as denoted by reaction formula (B) given below:xT1+LiT2yLi+yT2  (B)
Since the reaction in the second reaction et seq. given by reaction formula (B) does not proceed completely reversibly, Li is retained inside the alloy, and the lithium supply source is depleted if the charge-discharge cycle is repeated, which makes it impossible to further repeat the charge-discharge cycle. Incidentally, in the case of an amorphous alloy, the reaction proceeds smoothly in the initial stage. However, the crystallization of the amorphous alloy is promoted if the charge-discharge cycle is repeatedly carried out, with the result that the cycle deterioration is generated at the stage where the crystallization is promoted.
It should also be noted that the negative electrode material that carries out an alloying reaction with lithium in the charging stage exhibits a high reactivity with the nonaqueous electrolyte containing a nonaqueous solvent and, thus, a film of, for example, Li2CO3, is formed on the surface of the negative electrode as a result of the reaction carried out between lithium contained in the negative electrode material and the nonaqueous electrolyte. It follows that the Coulomb efficiency is lowered during the charge-discharge cycle. Further, in the case of using a positive electrode active material such as LiCoO2 as a lithium supply source, lithium in the supply source is depleted with progress in the charge-discharge cycle, with the result that a clear capacity deterioration is observed.
A nonaqueous electrolyte secondary battery comprising a negative electrode containing an alloy formed of at least two kinds of elements, the alloy having a hexagonal close-packed structure and a Ni2In type structure, is disclosed in, for example, Japanese Patent Disclosure (Kokai) No. 2001-250541. In this negative electrode, an element M1 such as tin or aluminum, which is capable of electrochemically carrying out an alloying reaction with lithium, is alloyed with lithium so as to charge the secondary battery. Therefore, lithium is stored within the alloy with progress in the charge-discharge cycle so as to decrease the lithium amount contributing to the charge-discharge operation. In addition, this negative electrode has a high reactivity with the nonaqueous electrolyte and, thus, the Coulomb efficiency is low during the charge-discharge cycle. It follows that the secondary battery disclosed in the prior art quoted above is incapable of obtaining a long charge-discharge cycle life.